1. Field of the Invention
The present invention relates to optical apparatuses for acquiring structure information and its processing method of optical interference signals, and particularly to an optical apparatus for acquiring structure information for acquiring structure information which, for example, can clearly visualize an image of muscularis mucosa and its processing method of optical interference signals.
2. Description of the Related Art
Recently, for example, in the field of medical care, optical coherence tomography (OCT) measurement has come to be used as a method of noninvasively obtaining a tomogram within a living body. OCT measurement has an advantage over ultrasonic measurement in that it has a one order of magnitude higher resolution of about 10 μm thereby enabling it to obtain a detailed tomogram within a living body. Moreover, it can obtain a three-dimensional tomogram by acquiring a plurality of images while shifting the position in the direction perpendicular to the tomogram.
Currently, there is need of acquiring a detailed tomogram of a living body for the purpose of cancer diagnosis and others. As a method for that, conventionally proposed is a “Time Domain OCT” in which light outputted from a low-coherence light source is scanned to obtain a tomogram for a subject (Japanese Patent Application Laid-Open No. 2000-131222.)
Moreover, recently a “Frequency Domain OCT” is utilized which is an improved OCT that has overcome the drawbacks of the “Time Domain OCT”, that is, a non-optimal signal-to-noise ratio, a low imaging frame rate, and a poor penetration depth (observation depth) (National Publication of International Patent Application No. 2007-510143 and “High-speed optical frequency-domain imaging,” Optics Express, Vol. 11, Issue 22, pp. 2953-2963.)
On the other hand, the Frequency Domain OCT is utilized in other diagnostics fields and widely applied to clinical cases.
Further, as for OCT tomographic images, there is disclosed a technique in which after performing preprocessing (smoothing and averaging processing, etc.) to image data, a differential filter is applied in the depth direction to determine the pixel position corresponding to the layer boundary (Japanese Patent Application Laid-Open No. 2008-206684 and Japanese Patent Application Laid-Open No. 2008-209166).
Examples of typical apparatus configurations for performing Frequency Domain OCT measurement include two types of apparatuses: an SD-OCT (Spectral Domain OCT) apparatus and an SS-OCT (Swept Source OCT) apparatus.
An SD-OCT apparatus is configured such that: using a wideband low-coherent light such as an SLD (Super Luminescence Diode) light source, an ASE (Amplified Spontaneous Emission) light source, and white light as the light source, the wideband low-coherent light is split into a sampling light and a reference light; thereafter the sampling light is irradiated onto a measuring object to cause the reflected light which returns in that occasion and the reference light to interfere with each other; then the interference light is decomposed into each frequency component using a spectrometer so that the interference light intensity for each frequency component is measured using a detector array in which elements such as photo diodes are arranged in an array; and a Fourier transformation of the resulting spectrum interference intensity signals is performed by a computer, thus making up an optical tomographic image.
On the other hand, an SS-OCT apparatus is configured such that a laser of which optical frequency is swept in time is used as the light source to cause a reflected light and a reference light to interfere with each other at each wavelength so that the temporal waveform of the signal corresponding to the temporal change of the optical frequency is measured, and Fourier transformation of the resulting spectral interference intensity signal is performed by a computer, thus making up an optical tomographic image.
By the way, although OCT measurement is, as described above, a method to acquire an optical tomogram of a specific region, under an endoscope, it can discern to what extent the cancer-affected portion infiltrates, for example, by finding out a cancer-affected portion through observation by a ordinary illumination light endoscope and a special light endoscope and performing OCT measurement of that region. Further, by two-dimensionally scanning the optical axis of the sampling light, it is possible to acquire three-dimensional information in conjunction with the depth information by OCT measurement.
By the fusion of OCT measurement and three-dimensional computer graphic technology, it becomes possible to display a three-dimensional structure model made up of structure information of a measuring object with a resolution of the order of micrometers, and therefore the three dimensional structure model by OCT measurement is hereafter referred to as an optical three-dimensional structure image.